JPH06268603A - Optical communication system - Google Patents

Abstract

PURPOSE: To provide a compact and simple optical communication system which improves its linearity and is suitably applied to the comparatively high frequency by producing the pre-compensation signals of opposite phases via the combination of an impedance element of proper value and a diode. CONSTITUTION: A capacitor 38 limits the current flowing through a solid state laser 42, and a resistance 22 secures the matching between the input impedance of an optical transmitter 8 and the output impedance of a signal voltage source. The degrees of pre-compensation can be set by a resistance 32. The voltage level rises between the 1st and 2nd terminals of a linearization device 25 like the rise of the input voltage level of the transmitter 8. A diode 30 is biased in the direction opposite to the direction of voltage change covering the device 25 and accordingly the voltage level covering the diode 30 is lowered. As a result, the impedance of the diode 30 increases and accordingly the overall impedance of the device 25 also increases. Thus, the voltage of the device 25 rises in a more proportional way and accordingly the voltage of the laser 42 rises with proportionality lower than that of the device 25. Such a compensating way is suitably applied to the nonlinearity of the super-linear solid state laser 42.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter which is connected to at least one optical receiver by a glass fiber, and which comprises an electro-optical converter and an electric input signal of the converter and an optical output signal thereof. The present invention relates to an optical communication system including an optical transmitter including linearization means for linearizing a relationship between parameters.

[0002]

2. Description of the Related Art A communication system of the above type is described in Electronics Letter 24, September 1992, Vol. 28, No. 28.
20, pp. 1875-1876, "Comparison Off Direct And External Modulation For CATV Lightwave Transmission At 1.5 .mu.m Wavelength".

In this communication system, an input signal is applied to an electro-optical converter which converts the input signal into a change in optical output parameters. This parameter is, for example, intensity, frequency, phase or a combination thereof. The electro-optical converter can include, for example, a laser or LED that is directly controlled by the input signal. Alternatively, the converter is a laser or LED that continuously emits an optical signal that is modulated by an external converter controlled by an input signal.
Can also be included.

One example of an input signal is a combination of multiple TV channels transmitted in frequency division multiplex mode such as in a cable television system. When so signaled, very strict requirements are placed on the linearity of the relationship between the input signal of the electro-optical converter and the corresponding parameter of its optical output signal. This is very important because the intermodulation in the transmission system is caused by non-linearity, so that the mixed product of the two TV channels (undesirable) can cause an interference signal on the third TV channel. Is.

[0005]

In known transmission systems, linearization means are used to improve this line shape, which is the inverse of the distortion at the input of the electro-optical converter which occurs in this converter. Do some pre-compensation. As a result, the relationship between the input signal of the electro-optical converter and the corresponding parameter of the optical output signal is more linear. This linearization means is a power division circuit,
It is rather complicated because it includes a pre-compensation circuit, a controllable delay line and a power combiner.

It is an object of the present invention to provide a transmission system of the type described above, the linearization means of which are significantly simpler. To this end, the invention is characterized in that the electro-optical converter is connected in series with a linearization means and the linearization means comprises a parallel circuit of a first circuit comprising a diode and a second circuit comprising an impedance element. And

Another embodiment of the invention is characterized in that the linearizing means is connected in parallel with the electro-optical converter and it comprises a series circuit of a first circuit comprising a diode and a second circuit comprising an impedance element. And

[0008]

[Operation] The combination of impedance element and diode is
Proper selection of the value of the impedance element produces an anti-phase pre-compensation signal in the same manner as the distortion introduced by the electro-optic converter, providing improved linearity. Since the linearization means includes only a small number of elements included in the control circuit of the electro-optical converter, the transmission system can be downsized and simplified. This miniaturization offers the additional advantage of making the transmission system suitable for relatively high frequencies.

The optical transmitter shown in US Pat. No. 4,952,820 includes a series circuit of a diode and an electro-optical converter. However, this does not include an impedance element to set the pre-compensation at the desired value, so the pre-compensation value is not correct.

Another embodiment of the invention is characterized in that this diode is a Schottky diode.

Since Schottky diodes have excellent high frequency performance, the use of Schottky diodes in the present invention provides excellent high frequency performance of transmission systems.
However, other types of diodes with good high frequency performance may be used.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the transmission system shown in FIG.
To N corresponding transmission signals S _{2 to} S _N to the respective first inputs.
Goes in. The frequency f ₂ is applied to the second input of each modulator 2 to N.
The signals of ~ f _N are input respectively. The outputs of modulators 2 to N are connected to the input of adder 6. The output of the adder 6 is connected to the input of the optical transmitter 8. The signal V _S is an input signal of the optical transmitter 8. The optical transmitter 8 is connected to a plurality of substations 12 to 18 by a glass fiber circuit 10.

The signals S _{2 to} S _N are video signals or data signals used in cable television, for example.
Modulator 2~N signal S ₂ to S _N respectively modulate their carriers at. This modulation can be amplitude modulation in various ways (eg A
M, VSB, SSB, ASK) or angle signals in various forms (eg FM, PM, FSK, PSK) by well-known modulation techniques. The frequency multiplexed signal V _S is obtained by taking the sum of the output signals of the modulators 2 to N. The optical transmitter 8 modulates this signal by the signal. Convert to an optical signal having parameters such as wavelength or intensity.

This optical signal is distributed by glass fiber circuit 10 to substations 12-18. Each of the substations 12 to 18 is provided with an optical receiver that converts the received optical signal into a frequency modulation signal V _S. The converted signal V _S is processed by a known means to be a signal suitable for a terminal such as a television receiver, a telephone set or a personal computer.

Intermodulation occurs when the optical transmitter imparts non-linearity to the modulated parameters of the input signal and the generated optical signal. For example, if two transmit signal modulates a carrier of a frequency f ₂ and f _3, the frequency f ₂ and f ₃ in the optical transmitter 8 to cause second-order distortion in the modulated parameters of the optical signal In addition to the components that they have, intermodulation components with frequencies f ₂ + f ₃ and | f ₂ −f ₃ | occur in the output signal of the optical receiver. If one of the other transmitted signals modulates a carrier having a frequency f ₂ + f ₃ or | f ₂ −f ₃ |, the reception of those transmitted signals is disturbed by these intermodulation signals. In order to keep this interference within reasonable limits, strict requirements should be placed on the linearity of the optical transmitter.

In the optical transmitter 8 shown in FIG.
Input signal enters the terminal. The second terminal of the resistor 20 is connected to the first unit of the linearizer 25.

The input of this linearizer is connected according to the invention to the first terminal of an impedance element, which here is a resistor 12. Linearizer 2
This input of 5 is connected to the cathode of Schottky diode 30 and the first terminal of resistor 24. Schottky diode 30 here forms the diode according to the invention.
The second terminal of the resistor 26 is connected to the first terminal of the capacitor 28. The ground of the Schottky diode 30 is connected to the first terminal of the resistor 34 and the first terminal of the resistor 32. The second terminal of the resistor 24 is connected to the positive terminal of the DC voltage source 36. Resistance 2
The second terminal of No. 4 and the negative terminal of the power supply 36 are connected to the reference potential point, that is, the ground point. The second terminal of the capacitor 28 and the resistor 3
The second terminal of 2 is connected to the output of the linearizer 25. Here, the first circuit includes a series circuit of a diode 30 and a resistor 32, and the second circuit includes a series circuit of a resistor 26 and a capacitor 28.

The output of the linearizer is the first of the capacitors 38.
Connect to the terminal. The second terminal of the capacitor 8 is connected to the first terminal of the resistor 40 and the first terminal of the direct current source 44. The second terminal of the resistor 40 is connected to the first terminal of the electro-optical converter, here a solid-state laser 42. Second solid-state laser 42
The terminal and the second terminal of the current source 44 are grounded.

Diode 30 is set to a quiescent current by voltage source 36 and resistors 34 and 24. This quiescent current is chosen so that the relationship between the voltage and current of diode 30 is effectively quadratic. Capacitors 22, 28, 3
Reference numeral 8 prevents direct current from flowing through the diode through a route other than the voltage source 36 and the resistors 24 and 34. The current source 44 provides a quiescent current to the solid-state laser 42. The capacitor 38 limits the current flowing through the solid-state laser 42, and the resistor 22 matches the input impedance of the optical transmitter 8 with the output impedance of the signal voltage source. The degree of pre-compensation can be set by the resistor 32.

The linearization effect of the present invention will be described assuming that the input voltage of the optical transmitter 8 increases. This rise causes a similar rise in the voltage between the first and second terminals of linearizer 25. Since diode 30 is biased in the opposite direction of this voltage change across the linearizer, the voltage across diode 30 is reduced. As a result, the impedance of the diode 30 increases and therefore the overall impedance of the linearizer 25 adds as well. As a result, the linearizer voltage rises more proportionally and the solid state laser 42 voltage therefore rises less proportionally. Such a compensation is suitable for compensating for the non-linearity of so-called superlinear solid-state lasers 42 in which the relevant parameters of the emitted light increase more proportionally to the applied signal. In order to compensate a sub-linear solid-state laser with an associated parameter of the transmitted light that increases with a low proportionality to the applied signal, the terminals of the diode 30 must be swapped and also the polarity of the DC voltage source 36 reversed. . By doing so, when the input voltage of the optical transmitter 8 rises, the impedance of the diode 30 is changed to the linearizer 25.
It is reduced by the increasing voltage across the linearizer 25, so that the increase in the voltage across the linearizer 25 is less proportional. As a result, the voltage across the solid-state laser 42 and resistor 40 combination rises more highly proportionally, thus allowing the sub-linear transfer of the solid-state laser 42 to be effectively compensated.

In the optical transmitter 8 shown in FIG. 3, the positive terminal of the DC power supply 46 is grounded. The negative terminal is connected to the first fixed terminal of the variable voltage divider 48 and the first terminal of the resistor 66. The second fixed terminal of the voltage divider 48 is connected to the first terminal of the resistor 54, the first terminal of the capacitor 50 and the first terminal of the capacitor 52 via the self-inductance 47. Capacitor 5
The second terminal of 0 forms the input of the optical transmitter to which the signal V _S is applied.

The second terminal of the capacitor 52 is connected to the first terminal of the resistor 55. The second terminal of resistor 55 is connected to the second terminal of resistor 54, the first terminal of capacitor 57, and the first terminal of linearizer 51. The second terminal of this linearizer is grounded. The linearization device 51 comprises in this case a series circuit of a Schottky diode 60 and an impedance element, here a resistor 56. The first circuit includes a Schottky diode 60 and the second circuit includes a resistor 56.

The second terminal of the capacitor 57 is connected to the first terminal of the resistor 58. The second terminal of the resistor 58 is connected to the fixed terminal of the electro-optical converter, here the cathode of the solid-state laser 62. The second terminal of resistor 58 is further connected to the first terminal of self-inductance 64. The second terminal of this inductance is connected to the second terminal of resistor 66.

DC voltage source 46 produces a quiescent current for solid state laser 62 and Schottky diode 60.
The quiescent current for the solid-state laser 62 is supplied via the resistor 66 and the self-inductance 64, and the quiescent current for the Schottky diode 60 is the variable voltage divider 48 and the resistors 54, 56.
Is supplied via. The capacitors 50 and 52 prevent the quiescent current of the Schottky diode 60 from flowing through the voltage source V _S (not shown) or the resistor 55.
The self-inductance 64 prevents the signal current from the voltage source V _S from flowing out through the resistor 66 and the voltage source 46. The self-inductance 47 prevents the signal source V _{S from} being short-circuited by the variable voltage divider 48.

The resistor 55 supplies current to the parallel circuit of the series circuit of the solid-state laser 62 and the resistor 58 and the linearization device 51 together with the required output impedance of the voltage source V _S. The degree of pre-compensation can be set by the resistor 56.

As the current I in resistor 55 increases, so does the solid state laser 62 and the linearizer current. This increasing current lowers the impedance of the linearizer and therefore increases its current more proportionally. As a result, the proportionality of the increase in the current of the solid-state laser 62 is reduced, thereby compensating for the super-linear characteristic of the solid-state laser 62. The degree of compensation can be set by the variable voltage divider and the resistor 56.

The optical transmitter shown in FIG. 4 is a modification of the one shown in FIG. It is so inserted between 1 terminal. Further, a capacitor 67 is added between the first terminal of the resistor 54 and the ground point. With these modifications, the linearizer for the signal voltage is connected in parallel as the solid-state laser 62 and resistor 58, but the polarity of the quiescent current is reversed.

If the current I in the resistor 55 increases, the current in the Schottky diode 60 decreases and therefore the impedance of the linearizer increases. Due to this increase in impedance, a relatively large portion of the current I is caused by the solid-state laser 6
2 and therefore its current increases more proportionally. Therefore, the circuit shown in FIG. 4 is suitable for linearizing the solid-state laser 62 having the sub-linear characteristic.

FIG. 5 (a) a shows the measured second and third order intermodulation products for a laser without a linearizer. This measurement was done by adding to the laser the sum of two carriers with a constant frequency difference of 47 MHz and amplitude-modulating these two carriers with a modulation depth of 35%. In FIG. 5A, the relative amplitudes of the second and third order intermodulation products are plotted against the frequency of each carrier. Figure 5
(A) clearly shows that the second-order distortion is dominant.

The result of this measurement is plotted in FIG. 5 (b) and the current of the linearizer is chosen so large that there is no large third order intermodulation by the linearizer.
The second-order intermodulation of the solid-state laser 62 is compensated. Figure 5
(B) shows that the second-order intermodulation is reduced by about 10 dB.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a transmission system to which the present invention is applied.

FIG. 2 is an optical transmitter 1 according to the first embodiment shown in FIG.
FIG.

FIG. 3 is an optical transmitter 1 according to a second embodiment shown in FIG.
3 is a circuit diagram showing the configuration of FIG.

FIG. 4 is a circuit diagram showing a configuration of an optical transmitter according to a third embodiment shown in FIG.

FIG. 5 is a graph showing intermodulation measurements in a transmission system without a linearizer and intermodulation measurements in a transmission system according to the present invention.

[Explanation of symbols]

 2 to N modulator 6 adder 8 optical transmitter 10 glass fiber circuit 12 to 18 substation 20, 24, 26, 32, 34 resistor 22, 28 capacitor 25, 51 linearizer 30, 60 Schottky diode 36, 46 DC Voltage source 42,62 Solid state laser 44 DC current source 48 Variable voltage divider 47,64 Self-inductance

Claims (6)

[Claims] 1. An optical transmitter connected by a glass fiber to at least one optical receiver, the optical transmitter comprising an electro-optical converter and an electrical input signal and an optical output signal of the electro-optical converter. An optical communication system including linearization means for linearizing a relationship between a parameter, wherein the electro-optical converter is connected in series with the linearization means, and the linearization means is a diode. An optical communication system comprising a parallel combination of a first circuit including the above and a second circuit including an impedance element. 2. An optical transmitter connected to at least one optical receiver by a glass fiber, the optical transmitter including an electro-optical converter and an electrical input signal and an optical output signal of the electro-optical converter. An optical communication system including linearization means for linearizing the relationship between the linearization means, the linearization means being connected in parallel to the electro-optical converter, and the linearization means including a diode. An optical communication system comprising a series combination of a first circuit and a second circuit including an impedance element. 3. The optical communication system according to claim 1, wherein the diode includes a Schottky diode. 4. An electro-optical converter and linearizing means for linearizing a relationship between an electrical input signal and an optical output signal of the converter, the electro-optical converter being the linearizing means. An optical transmitter characterized in that it is connected in series and this linearization means comprises a parallel combination of a first circuit including a diode and a second circuit including an impedance element. 5. An electro-optic converter and linearization means for linearizing the relationship between the electrical input signal and the optical output signal of the converter, the linearization means comprising the electro-optic converter. And a series combination of a first circuit including a diode and a second circuit including an impedance element connected in parallel. 6. The optical transmitter according to claim 4, wherein the diode includes a Schottky diode.